1. Field of the Invention
The present invention relates to a control apparatus equipped with a gas sensor which is used, for example, for controlling an air-fuel ratio of an internal combustion engine.
2. Description of the Related Art
In general, a gas sensor including a gas element is disposed in an exhaust system of an internal combustion engine for use in controlling the engine's air-fuel ratio. A widely known type of gas sensor element includes a detection element constituted by an oxygen-ion conductive solid electrolyte (e.g., zirconia) and electrodes formed on the surface thereof, and a heater for heating the detection element to an activation temperature or higher temperature. The heater generates heat when electrical power is supplied thereto from an external power source, and its heat generation quantity can be controlled by controlling the supply of electricity to the heater.
In order to accurately detect a specific gas, electricity is desirably supplied to the heater in such manner that the temperature of the detection element is rapidly increased, to thereby quickly activate the gas detection section of the detection element. However, under conditions where the temperature of the interior of the exhaust pipe (the temperature of the wall surface of the exhaust pipe) is low (e.g., at the time of cold start of the engine) and moisture contained in exhaust gas discharged from the engine condenses within the exhaust pipe; that is, under conditions where condensate is produced within the exhaust pipe, and when the detection element is heated to a high temperature by supplying electricity to the heater, a phenomenon called “splash water cracking” may occur. That is, the gas sensor element may crack due to thermal shock when condensation water adheres to the gas sensor element to quickly cool the same.
In view of the above, in order to suppress splash water cracking of a gas sensor element, a technique has been proposed for determining, using a temperature detection means attached to the wall surface of the exhaust pipe, whether or not water is present within the exhaust pipe, and limiting the supply of electricity to the heater when water is detected within the exhaust pipe (see, for example, Patent Document 1). In this technique, when it is determined that condensation water is being generated within the exhaust pipe, the supply of electricity to the heater is restricted so as to mitigate thermal shock imparted to the gas sensor element when condensation water adheres to the gas sensor element. When it is determined that condensation water is not being generated within the exhaust pipe, the restriction on the supply of electricity to the heater is removed, and main electrification is begun in order to activate the detection element.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2001-41923.
                3. Problems to be Solved by the Invention        
However, through studies performed by the present inventors, it was found that the phenomenon of cracking of a gas sensor element cracking is observed not only in the case where condensation water adheres to the gas sensor element while being heated to high temperature, whereby the gas sensor element is subjected to thermal shock resulting in splash water cracking, but also in the case where water has adhered to the detection element before start of supply of electricity to the heater or where condensation water adheres to a detection element which has been heated to a temperature of less than 100° C. (hereinafter referred to as “condensation water cracking”).
That is, at the time of cold start of the internal combustion engine, the gas sensor element itself is cold, and condensation water may adhere to the outer surface of the detection element from the beginning of cold start. When supply of electricity to the heater is commenced in this state, the temperature of the heater increases continuously. However, because of the condensation water adhering to the surface of the detection element, the temperature of the detection element does not increase beyond about 100° C. until the condensation water evaporates. If there is a large difference in temperature between the heater and the detection element, at the instant when the condensation water adhering to the surface of the detection element has completely evaporated, the temperature of the detection element abruptly increases. This is due to influence of the heater, whereby cracking (condensation water cracking) may be generated in the detection element due to thermal shock.
The technique of the above-mentioned Patent Document 1 restricts supply of electricity to the heater when it is determined that condensation water is being generated within the exhaust pipe. However, Patent Document 1 does not describe the temperature at the time when restriction on the supply of electricity to the heater is removed. Therefore, according to this technique, supply of electricity to the heater may be restricted in such manner that the temperature of the detection element is maintained in a region below 100° C. However, in some cases the above-described condensation water cracking is induced when the mode of supply of electricity to the heater is switched to main electrification after having been restricted to maintaining the temperature of the detection element below 100° C. Specifically, in the case where water adheres to the surface of the detection element before start of supply of electricity to the heater, the above-described heater control causes the following phenomenon. Although the temperature of the heater increases continuously after the start of main electrification, the temperature of the detection element does not quickly elevate and exceed 100° C., with resultant occurrence of condensation water cracking.